Fluorescent colorants are well known and represent an important class of materials commonly used in coloring printing inks, paints and plastics to impart a desired color. For example, the prior art describes a number of fluorescent organic dyes; see for example U.S. Pat. Nos. 2,929,931; 2,938,292; 2,983,686; 3,105,908; 3,115,417; 3,162,642; 3,177,153; 3,407,196; 3,420,821; 3,423,407; 3,492,478; 3,560,238; 3,597,364; 3,599,389; 3,614,430; 3,671,451; 3,760,161; 3,776,761; 3,892,972; 3,960,755; 4,006,158; 4,015,131. These include, stilbenes (such as diamino-stilbene-di-sulfuric acid), berberin sulfate tetra-ethyl-diamino-o-carboxyphenyl, xanthenyl chloride, tetramethyl-di-amino-dephenyl-ketoamine hydrochloride, auranin, xylene red B, rhodamine B, rhodamine 6G, and fluorescein.
The fluorescent organic dyes in the prior art are also used in marking applications such as machine sorting, mail sorting and web registration control in manufacturing operations. For some applications it is desired that the marking be colorless when viewed under visible light and emit visible radiation when irradiated by an ultraviolet incident light source. In these particular applications, however, it is not sufficient that the marking only emit radiation when irradiated, the marking must emit radiation at a different and higher wavelength than the incident light and at a level sufficient to be detected by a detector such as that described in U.S. Pat. No. 4,467,207. Also, it is equally important that the markings do not bleed or migrate through the web or substrate so as to enlarge or relocate the marking.
U.S. Pat. No. 4,467,207 describes various fluorescent organic dye compounds that do not migrate or bleed when used in markings for thermoplastic web applications. These include coumarin dyes such as 4-methyl-7-(sulfo methyl amino) coumarin sodium salt, benzoxazoles, stilbene dyes such as Phorite CL, Phorite BA (available from Verona Dyestuff Division, Mobay Chemical Corporation), TH-40 (available form Sandoz Corporation, East Hanover, N.J.), and IR-125, a dark red organic laser dye (available from Eastman Kodak Corporation, Rochester, N.Y.).
As previously mentioned, fluorescent organic dyes suffer from migration or bleeding. This not only presents a problem in marking applications but lessens the effect of the color. In addition, this becomes a serious health concern where the fluorescent organic dyes are used in food packaging applications or in any application where the dye may come in contact with the skin. In general, organic dyes also have low thermal stability, thereby precluding their use at high temperatures.
In order to overcome some of the practical limitations in using fluorescent organic dyes, fluorescent pigments, which represent another class of colorants, are sometimes employed. Broadly speaking, the essential difference between a organic dye and a organic pigment is solubility. Conventional fluorescent pigments,(actually made of organic dyes are dissolved in a resin matrix, which resin inturn may not be soluble in the application medium), tend to suffer from migration and plateout. For example, along with being thermally unstable, organic dyes are fairly soluble in plastics while pigments are insoluble and have higher thermal stability. The prior art describes only a limited number of fluorescent organic pigments. No fluorescence is found, for example, in commonly used organic pigments such as Pigment Red 188, Pigment Red 202 and Pigment Orange 36.
U.S. Pat. No. 5,095,056 teaches a heat stable Pigment Red 202 having increased crystalite size. However, it reports increased crystalite size can be achieved only when the pigment is substantially fluorescence free. Thus, the prior art teaches that an increase in the particle size of an organic pigment is accompanied by a decrease in fluorescence and vice versa.
U.S. Pat. Nos. 4,370,269 and 4,476,052 describe pigments having improved hiding power such as Pigment Orange 36 but neither teaches that the pigment possesses any fluorescent properties.
U.S. Pat. Nos. 3,162,642 and 4,983,661 describe a "fluorescent pigment" used in marking applications. However, these pigments are classical organic dyes dissolved in a resin matrix. The resin matrix is then formed to a specific size, typically of several microns, so that it may be used as a pigment. Organic pigments of this type are further described by P. A. Lewis, Pigment Handbook, on pages 860-61.
Herbst and Hunger, Industrial Organic Pigments, on pages 555-556, describe a fluorescent Pigment Yellow 101, a disazomethine compound. However, this compound is not in the same class of disazo pigment compounds as the present invention.
Yellow azo pigments represent an important class of organic pigments and include monoazo yellow and disazo yellow pigments. Yellow azo pigments are well known and are typically derived by coupling a acetoacetanilide or pyrazolone derivatives with a diazonium or bis-diazonium salt of an aromatic amine or diamine. A number of patents are available which describe the preparation and various uses for these pigments; see for example U.S. Pat. Nos. 2,341,889; 3,617,171; 3,759,731; 3,775,148; 3,776,749; 4,341,701; 4,968,352; and 5,352,280. These pigments are not commonly known to exhibit fluorescence when exposed to incident radiation.
While not wishing to be bound by theory, it is believed that the chemical structure and physical form of a pigment greatly influences whether or not it will be fluorescent. P. A. Lewis, Pigment Handbook, on pages 861-879, provide a detailed discussion on the theory of fluorescence dye based organic pigments. The properties of commercial grade organic pigments depend, in most instances, on process treatments which are used extensively to generate the various pigment grades for select applications. These treatments, may for example, involve chemical reactions which modify particle size, crystallite size, distribution and overall crystal morphology of the pigment. These all effect such properties such as shade, color strength, dispersibility, rheology, gloss, etc. It is also known that heat treating a pigment, for example, in a slurry, can cause crystalite growth. No where in the prior art, however, does it indicate that such modifications would impart fluorescence.
It is an object of the present invention to provide fluorescent yellow azo pigments.
It is an object of the present invention to provide fluorescent yellow azo pigments of increased crystallite size.
It is a further object of the present invention to provide fluorescent yellow azo pigments that do not migrate or bleed from inks to thermoplastic webs or substrates.
It is yet a further object of the present invention to provide printing inks, coatings and polyolefins that contain, as a component, the fluorescent yellow azo pigments of the present invention.
Other objects and advantages of the invention will become apparent from the following description of embodiments.